dq2-get, retry failed transfers

Description

When download problems occur, dq2-get needs to retry the download.

Because of the way dq2-get has been implemented with threads, this is not trivial to do. It has to be done after all the file threads have completed, the list of successes needs to be examined from dq2-get, retried by enacting a threading pool, and then that list modified for the summary screen.